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Abstract:

A plasma spray method for the manufacture of an ion conductive membrane
is provided which ion conductive membrane has an ion conductivity, in
which method the membrane is deposited as a layer (11) onto a substrate
(10) in a process chamber, wherein a starting material (P) is sprayed
onto a surface of the substrate (10) in the form of a process beam (2) by
means of a process gas (G), wherein the starting material is injected
into a plasma at a low process pressure, which is at most 10,000 Pa, and
is partially or completely molten there. Oxygen (O2; 22) is supplied
to the process chamber (12) during the spraying at a flow rate which
amounts to at least 1%, preferably at least 2%, of the overall flow rate
of the process gas.

Claims:

1. A plasma spray method for the manufacture of an ion conductive
membrane which has an ion conductivity, in which method the membrane is
deposited as a layer (11) onto a substrate (10) in a process chamber,
wherein a starting material (P) is sprayed onto a surface of the
substrate (10) in the form of a process beam (2) by means of a process
gas (G), wherein the starting material is injected into a plasma at a low
process pressure, which is at most 10,000 Pa, and is partially or
completely molten there, characterized in that oxygen (O2; 22) is
supplied to the process chamber (12) during the spraying at a flow rate
which amounts to at least 1%, preferably at least 2%, of the overall flow
rate of the process gas.

2. A method in accordance with claim 1, in which the plasma defocuses and
accelerates the process beam (2).

3. A method in accordance with claim 1, in which the process pressure in
the process chamber (12) is set to a value of at least 50 Pa and at most
2000 Pa.

4. A method in accordance with claim 1, in which the starting material
(P) is a powder whose chemical composition is substantially the same as
the chemical composition of the layer (11).

5. A method in accordance with claim 1, in which the starting material
(P) is a powder whose phase composition is substantially the same as the
phase composition of the layer (11).

6. A method in accordance with claim 1, in which the layer (11) forming
the membrane is composed of a ceramic material which is an oxide of the
perovskite type.

7. A method in accordance with claim 1, in which the layer is composed of
a perovskite which includes lanthanum (La), strontium (Sr), cobalt (Co)
and iron (Fe).

8. A method in accordance with claim 1, wherein the overall flow rate of
the process gas on plasma spraying is smaller than 200 SLPM and in
particular amounts to 100 to 160 SLPM.

9. A method in accordance with claim 1, wherein the process gas is a
mixture of argon and helium.

10. A method in accordance with claim 1, in which the process gas is
composed of argon, helium and hydrogen.

11. A method in accordance with any one of the preceding claims claim 1,
wherein the layer (11) produced on the substrate (10) has a thickness of
less than 150 micrometers and preferably of 20 to 60 micrometers.

12. A method in accordance with claim 1, in which the process beam (2) is
swiveled or scanned relative to the surface of the substrate (10).

13. A method in accordance with claim 1, in which the ion conductive
membrane is an oxygen permeable membrane which has an ion conductivity
for oxygen.

14. An ion conductive membrane, in particular an oxygen permeable
membrane manufactured according to a method in accordance with claim 1.

Description:

[0001] The invention relates to a plasma spray method for the manufacture
of an ion conductive membrane in accordance with the preamble of the
independent patent claim.

[0002] Ion conductive membranes are membranes which have a high selective
permeability for specific ions, such as, for example, oxygen-permeable
membrane layers which have a high selective permeability for oxygen and
are substantially impermeable for other gases. Correspondingly such
membranes are used in order to extract or to purify oxygen from gas
mixtures or fluid mixtures.

[0003] Such membranes can be manufactured from the most diverse materials,
for example, they can be composed of complex oxide materials which have a
specific chemical composition and which form specific phases. In
particular ceramic membranes are known which are composed of oxides of
the perovskite type and which are manufactured in the form of thin,
dense--this means not porous--layers. Such membranes, for example, have
both an ion conductivity for oxygen and also an electron conductivity.

[0004] A material which is investigated and used today for the
manufacture, in particular of oxygen-permeable membranes, is a ceramic
which has a perovskite structure and includes the elements lanthanum
(La), strontium (Sr), cobalt (Co) and iron (Fe) beside oxygen. According
to the first letters of these four elements the substance is typically
referred to as LSCF.

[0005] Oxygen-permeable membranes or generally ion conductive membranes
made of such materials can be manufactured, for example, by means of
conventional manufacturing techniques for ceramics, such as for example
pressing, tape casting, slip casting or sintering, or also by means of
thermal spraying. For the latter, in particular thermal spray processes
are suitable which are carried out in vacuum, this generally means that
the spray process is carried out at a process pressure which is smaller
than the environmental pressure (normal air pressure).

[0006] In particular a thermal low pressure plasma spray method or a
vacuum plasma spray method, which is referred to as an LPPS method (Low
Pressure Plasma Spraying) is suitable. By means of this vacuum plasma
spray method particularly thin and dense layers can namely be sprayed,
i.e. such which are required also for ion conductive membranes or
oxygen-permeable membranes.

[0007] In practice it has now been shown that on vacuum plasma spraying of
such membranes the chemical composition of the layer manufactured by way
of spraying no longer corresponds to the chemical composition of the
starting material, so that also the generated layer no longer has the
desired chemical composition, or that the phase composition of the layer
no longer is the same as that of the starting material. Thus, for
example, it can be seen for perovskite substances that the desired
phase--in this case thus the perovskite phase--is no longer formed or is
only formed to a lesser degree. Specifically, the condensation of
metallic elements, such as for example, iron or cobalt, at the walls of
the process chamber can be monitored.

[0008] For this reason it is an object of the invention to solve this
problem and to provide a plasma spray method in which an ion conducting
and specifically oxygen-permeable membrane of an improved quality can be
manufactured.

[0009] The subject matter of the invention solving this object is
characterized by the independent method claim.

[0010] In accordance with the invention, thus a plasma spray method for
the manufacture of an ion conductive membrane is provided, which has an
ion conductivity, in which method the membrane is deposited as a layer
onto a substrate, wherein a starting material is sprayed onto a surface
of the substrate in the form of a process beam by means of a process gas,
wherein the starting material is injected into a plasma at a low process
pressure, which is at most 10,000 Pa and is partially or completely
molten there. Oxygen is supplied to the process chamber during the
spraying at a flow rate which amounts to at least 1%, preferably at least
2%, the overall flow rate of the process gas.

[0011] Preferably, an inert atmosphere or an atmosphere with reduced
oxygen content is present during the spraying in the process chamber.

[0012] It has been shown that one can counter-act the undesired chemical
changes of the starting material during the thermal spraying by the
measure of supplying of oxygen, whereby both the chemical composition of
the layer generated by way of spraying and also its phase, corresponds to
the desired composition. Through the supply of oxygen during the thermal
spraying it is efficiently avoided that the formation of an atmosphere
with reducing properties arises on thermal spraying in the process
chamber.

[0013] Thereby it is, for example, avoided that metal oxides contained in
the starting material are reduced and are deposited in the form of
elemental methods or in the form of combinations thereof at the walls of
the process chamber. In particular the deposition of a metallic cobalt or
of iron and their combination can be avoided or at least significantly
reduced on spraying of an LSCF powder, so that ion conducting and
particularly oxygen-permeable membranes of improved quality can be
manufactured.

[0014] Preferably, the membrane also has an electron conductivity beside
its ion conductivity.

[0015] Preferably, the plasma spray process is carried out such that the
plasma defocuses and accelerates the process beam. By means of this
method particularly thin and dense layers can be advantageously made.

[0016] In practice it has been found to be advantageous when the process
pressure in the process chamber is set to a value of at least 50 Pa and
to at most 2000 Pa.

[0017] Particularly preferably, the method is carried out such that the
starting material is a powder whose chemical composition is substantially
the same as the chemical composition of the layer, this means that a
powder is used as a starting material which substantially has the same
chemical composition which the sprayed layer should also have.

[0018] Furthermore, it is preferred to carry out the method such that the
starting material is a powder whose phase composition is substantially
the same as that of the phase composition of the layer.

[0019] In a preferred embodiment the layer forming the membrane is
composed of a ceramic material which is an oxide of the perovskite type.

[0020] In view of the oxygen permeability it has been particularly proven
when the layer is made of a perovskite which includes lanthanum (La),
strontium (Sr), cobalt (Co) and iron (Fe). It is naturally understood
that the term "composed of" in this connection means that the substantial
part of the layer is present in the form of a perovskite phase. Naturally
it is also possible that also other phases are present in this layer to a
smaller degree.

[0021] In practice it has been proven when the overall flow rate of the
process gas is less than 200 SLPM on plasma spraying and, in particular
amounts to 100 to 160 SLPM (SLPM: standard liter per minute).

[0022] In a first preferred embodiment of the method, the process gas is a
mixture of argon and helium.

[0023] In a second preferred embodiment of the method, the process gas is
composed of argon, helium and hydrogen.

[0024] Preferably, the plasma spray method is carried out such that the
layer generated on the substrate has a thickness less than 150 Micrometer
and preferably of 20 to 60 Micrometers. This layer thickness has proven
itself for the oxygen-permeable membrane.

[0025] It has also been found advantageous when the process beam is
pivoted or is scanned relative to the surface of the substrate. This can,
for example, take place by pivoting the plasma generator and/or the
plasma source and/or the exit nozzle. The process beam is thus guided
relative to the substrate so that the substrate is scanned, i.e. is
covered one or more times by the process beam. Alternatively or in
addition hereto it is naturally also possible to move the substrate.
There are naturally many possibilities of realizing this relative
movement between the process beam and the substrate. This pivot movement
and/or the scanning of the substrates causes that the oxygen introduced
into the process chamber comes into contact as much as possible with the
process beam or with the layer building up on the substrate.

[0026] The method is particularly suitable also for the case of
application in which the ion conductive membrane is an oxygen-permeable
membrane which has an ion conductivity for oxygen.

[0027] By means of the invention, an ion conductive membrane is further
provided, in particular an oxygen-permeable membrane which is
manufactured in accordance with the method according to the invention.

[0028] Further advantageous measures and preferred embodiments of the
invention results from the dependent claims.

[0029] In the following, the invention will be explained in detail by way
of embodiments and with reference to the drawings. In the schematic
drawing shown partially in section, there is shown:

[0030]FIG. 1 a schematic illustration of an apparatus for carrying out a
method in accordance with the invention.

[0031] The plasma spray method in accordance with the invention for
manufacturing an ion conductive membrane will be explained in the
following with reference to a case of application particularly relevant
for practice in which the membrane is a membrane which is selectively
permeable for oxygen, which thus has an ion conductivity for oxygen.
Preferably, the membrane also has an electron conductivity. The method is
a thermal spray method which is carried out in vacuum, i.e. at a process
pressure which is smaller than this environmental pressure.

[0032]FIG. 1 shows a plasma spray apparatus in a very schematic
illustration which is referred to overall using the reference numeral 1
and which is suitable for carrying out a method in accordance with the
invention. Moreover, a substrate 10 is schematically illustrated in FIG.
1 onto which an oxygen-permeable membrane is deposited in the form of a
layer 11. Furthermore, a process chamber 12 is indicated in which the
method is carried out.

[0033] The method in accordance with the invention includes a plasma spray
method, the kind of which is described in WO-A-03/087422 or also in U.S.
Pat. No. 5,853,815. This plasma spray method is a thermal spray method
for the manufacture of a so-called LPPS-thin film (LPPS=Low Pressure
Plasma Spraying).

[0034] Specifically, an LPPS-based method is carried out in the plasma
spray apparatus 1 illustrated in FIG. 1. In this a conventional LPPS
plasma spray method is modified in view of the process technology,
wherein a space flooded by plasma ("plasma flame" or "plasma beam") is
expanded and extended to a length of up to 2.5 m due to the changes. The
geometric extent of the plasma leads to a uniform expansion--a
"defocusing"--and to an acceleration of a process beam which is injected
into the plasma with a feed gas. The material of the process beam which
is dispersed to a cloud in the plasma and is partially or completely
molten there arrives uniformly distributed at the surface of the
substrate 10.

[0035] The plasma spray apparatus 1 illustrated in FIG. 1 includes a
plasma generator 3 known per se having a non-closer illustrated plasma
burner for the generation of a plasma. In a manner known per se, a
process beam 2 is generated with the plasma generator 3 from a starting
material P, a process gas and/or a process gas mixture G and electrical
energy E. The injection of these components E, G and P is symbolized by
the arrows 4, 5, 6 in FIG. 1. The generated process beam 2 exits through
an exit nozzle 7 and transports the starting material P in the form of
the process beam 2 in which the material particles 21 are dispersed in
the plasma. This transport is symbolized by the arrows 24. The material
particles 21 are generally powder particles. The morphology of the layer
11 deposited on the substrate 10 is dependent on the process parameters
and, in particular on the starting material P, the process enthalpy and
the temperature of the substrate 10. Preferably, the plasma generator 3
and/or the plasma torch is pivotable with regard to the substrate 10 as
is indicated by the double arrow A in FIG. 1. Therefore the process beam
2 can be moved to and fro in a pivot movement over the substrate 10.

[0036] In the LPPS process described in this context, the starting
material P is injected into a plasma defocusing the material beam and is
partially or completely molten or at least made plastic there at a low
process pressure of at most 10,000 Pa and preferably of at least 50 Pa
and at most 2000 Pa. For this purpose a plasma having a sufficiently high
specific enthalpy is generated, so that a very dense and thin layer 11
emerges at the substrate. The variation of the structure is substantially
influenced and controllable through the coating conditions, in particular
of process enthalpy, work pressure in the coating chamber as well as the
process beam. Therefore the process beam 2 has properties which are
determined by the controllable process parameters.

[0037] For the manufacture of the oxygen-permeable membrane the layer 11
is generated such that it has a very dense microstructure.

[0038] Initially the method step of generating the layer 11 by means of
LPPS will now be explained in detail.

[0039] A powder of suitable composition is selected as starting material P
as will be explained later on in detail. As was already mentioned the
plasma flame is very long in the LPPS process in comparison to
conventional plasma spray methods due to the set process parameters.
Furthermore, the plasma flame is strongly expanded. A plasma with a high
specific enthalpy is generated, whereby a high plasma temperature
results. Due to the high enthalpy and the length and/or the size of the
plasma flame, a very high influx of energy into the material particles 21
is brought about which are thereby, on the one hand, strongly accelerated
and, on the other hand, are brought to a very high temperature, so that
they are very well melted and are also still very hot after their
deposition on the substrate 10. Since, on the other hand, the plasma
flame and therefore the process beam 2 is very strongly expanded, the
local heat flow into the substrate 10 is small, so that a thermal damage
of the material is avoided. The expanded plasma flame further has the
effect that, typically on the one time covering of the substrate 10 with
the process beam 2, the material particles 21 are deposited in the form
of individual splats which do not manufacture a continuous, this means
connected layer. Thereby, very thin layers 11 are manufacturable. The
high kinetic and thermal energy which the material particles obtain
during their long stay in the plasma flame in comparison to conventional
plasma spray methods promote the formation of a very dense layer 11
which, in particular has very few boundary surface hollow spaces between
splats lying on top of one another.

[0040] The plasma is, for example, generated in the plasma torch in the
plasma generator 3 generally known per se with an electric direct current
and by means of a pin cathode, as well as a ring-like anode. The power
consumption of the plasma torch lies in the region of up to 180 kW. The
power supplied to the plasma, the effective power can be empirically
determined with regard to the resulting layer structure. The effective
power which results through the difference between the electric power and
the heat led away by cooling from experience lies in the region, for
example, of 40 to 130 kW, in particular of 80 to 100 kW. In this
connection it has been proven that if the electric current for the plasma
generation lies between 1000 and 3000 A, in particular between 1500 and
2600 A.

[0041] A value of between 10 and 10,000 Pa is selected for the process
pressure of the LPPS-TF plasma spraying for the generation of the
oxygen-permeable membrane in the process chamber 12, preferably between
50 and 2000 Pa.

[0042] The starting material P is injected into the plasma as a powder
jet.

[0043] The process gas for the generation of the plasmas is preferably a
mixture of inert gases, in particular a mixture of argon Ar, of helium He
and possibly of hydrogen H. In practice the following gas flow rates have
been particularly proven for the process gas:

[0044] Ar-flow rate: 30 to 150 SLPM, in particular 50 to 100 SLPM;

[0045] H2-flow rate: zero to 20 SLPM, in particular 2 to 10 SLPM;

[0046] He-flow rate: zero to 150 SLPM, in particular 20 to 100 SLPM;

[0047] wherein the overall flow rate of the process gas is preferably
smaller than 200 SLPM and in particular amounts to 100 to 160 SLMP.

[0048] In accordance with the invention oxygen is supplied to the process
chamber 12 during the thermal spraying such as it is indicated by the
arrow referred to with the reference numeral O2 and the oxygen particles
22. In this respect the oxygen particles 22 are brought into contact with
the process beam 2 and/or with the substrate 10 and/or with the layer 11
being built up thereon. The oxygen is supplied to the process chamber at
a flow rate which amounts to at least 1%, preferably to at least 2% of
the overall flow rate of the process gas. The oxygen particles 22 mix
with the process beam 2 and are also present in the vicinity of the
substrate 10 and/or of the layer being built up thereon. Thereby it is
ensured that the different components of the starting material P are
mixed through with the oxygen particles 22 during their transport in the
process beam 2 and/or after their deposition on the substrate 10 are
present in their vicinity. The oxygen particles 22 prevent the build up
of a reducing atmosphere, which could, for example, reduce metal oxides
to elemental methods or other reduction products and connections of the
starting oxides in the process beam 2 or at the surface of the substrate
10. Therefore the supplied oxygen efficiently prevents the undesired
reduction of components of the starting material P. In order to enable an
as good as possible contact between the process beam and/or the layer 11
building up itself, on the one hand, and the oxygen particles 22 it is
advantageous to pivot the plasma generator 3 and/or the plasma torch with
regard to the surface of the substrate 10 to be coated.

[0049] It can be advantageous when the substrate--additionally or
alternatively--is moved during the material deposition by means of rotary
movements or pivot movements relative to this cloud.

[0050] In the following, reference is made to the example particularly
relevant for practice in which the oxygen-permeable membrane is composed
of a ceramic, which includes the elements lanthanum (La), strontium (Sr),
cobalt (Co) and iron (Fe) beside oxygen. Such ceramics are referred to as
LSCF. In this respect it is desired that the membrane is almost
completely made up of a perovskite structure. However, it is naturally
understood that the invention is not limited to such substances, but is,
in particular also suitable for other ceramic materials, specifically
oxides of the perovskite type.

[0051] As already mentioned, the starting material P is provided in the
form of a powder. The plasma spray method is then carried out so that the
chemical composition of the layer is substantially the same as the
chemical composition of the starting material.

[0052] LSCF as a ceramic material belongs to the oxides of the perovskite
type, which substantially have the form ABO3. In this respect A
stands for LaxSr1-x and B for CoyFe1-y. However, it
should be noted that the stoichiometry does not have to be exactly
satisfied. It is rather possible that the La content and the Sr content
and/or the Co content and the Fe content do not have to match exactly to
one. Also the oxygen content can deviate from the precise stoichiometry.
For this reason, it is typical to state the oxygen content as 3-σ,
wherein σ is the deviation of the oxygen content from the
stoichiometric equilibrium. The minus signs indicates that this deviation
generally is a deficiency of oxygen, this means that the oxygen is
present under stoichiometrically.

[0053] In the example described here, LACF is present in the form
La0.58Sr0.4Co0.2Fe0.8O3-σ. The starting
material P is present as powder. For the manufacture of the powder
particles different methods can be used, for example, spray drying or a
combination of melting and subsequent braking and/or milling of the
solidified melt.

[0054] The manufacture of such powders is generally known and does not
require a detailed explanation here. In view of the plasma spraying it is
preferred, when the powder seeds have a size of, for example, from
25±5 μm.

[0055] The value σ for the deviation of the oxygen content in the
stoichiometry is, for example, 0.3.

[0056] For the two examples described in the following
La0.58Sr0.4Co0.2Fe0.8O3-σis respectively
used as a starting material. The process pressure in the pressure chamber
12 is set to a value between 50 and 2000 Pa. By means of a plasma torch,
which can generate a plasma of high specific enthalpy of up to 10,000 to
15,000 kJ/kg and which can take up a power of up to 180 kW, a plasma beam
and/or a process beam 2 of high enthalpy is generated. The process beam 2
has a length of 1000 to 2000 mm and a diameter of up to 200-400 mm. The
length of the process beam 2 substantially corresponds to the spray
distance, this means to the distance D between the exit nozzle 7 and the
substrate 10. A porous plate of a high temperature nickel-based alloy
serves, for example, as a substrate or a substrate of a fire-resistant
ceramic.

[0057] The starting material P is introduced by means of two powder
supplies, wherein the feed rate is up to 120 g/min, typically 40 g/min.
By means of a pivot movement of the plasma torch a very thin and dense
layer 11 is applied onto the substrate 10, wherein the high energy input
into the material particles 21 and the high (ultrasonic) speed in the
process beam 2 enables a very dense build up of the layer 11. The layer
11 is sprayed until it finally has a layer of 20-60 μm. The coating
time amounts to approximately one minute. During the thermal spraying,
the process chamber 12 is supplied with oxygen and indeed at a flow rate
of at least 1%, preferably with at least 2% of the overall flow rate of
the process gas. Hereby, the reduction and the degradation of the
starting material P and/or its components is avoided or is at least
strongly reduced. In particular the precipitation and/or the deposition
of elementary Co or Fe or of their combinations is avoided or at least
strongly reduced. From this, it results that the chemical and
phase-compositions of the layer 11 are substantially the same as that of
the starting material P.

EXAMPLE 1

[0058] The process is carried out as described above. A mixture of argon
and helium is used as a process gas, wherein the argon flow rate amounts
to 80 SLPM and the He flow rate amounts to 40 SLPM, so that the overall
flow rate of the process gas amounts to 120 SLPM. The current for the
generation of the plasma amounts to 2600 A.

EXAMPLE 2

[0059] The process is carried out as described above. A mixture of argon,
helium and hydrogen is used as a process gas, wherein the Ar flow rate
amounts to 80 SLPM, the He flow rate amounts to 20 SLPM and the H2
flow rate amounts to 6 SLPM, so that the overall flow rate of the process
gas amounts to 106 SLPM. The current for the generation of the plasma
amounts to 2600 A.

[0060] In both cases oxygen-permeable membranes result whose chemical
composition and perovskite phase structure is substantially the same as
that of the starting material.